This invention relates to compression encoding of digital video signals and, more particularly, to a technique for providing highly efficient encoding using orthogonal transformation, such as discrete cosine transformation.
Digital video recorders, such as digital VTRs, have been developed for recording a digitized video signal on a magnetic medium. Since the bandwidth of a digital video signal is quite wide, it is difficult to record a digital video signal directly on a video tape. Hence, techniques have been proposed for encoding the digital video signal in a manner which reduces its bandwidth. So-called compression encoding techniques include the orthogonal transformation of the digital video signal; and one highly efficient encoding technique utilizes discrete cosine transformation, or DCT. Such encoding also is quite useful in digital video transmission.
When compression encoding a digital video signal using a DCT transform, a frame or field of picture elements is segmented into blocks, sometimes referred to as DCT blocks, formed of, for example, an 8.times.8 array of picture elements. These picture elements are presented as information on a time axis, and DCT transformation transforms this information into data along a frequency axis. That is, DCT-transformed video data generally is represented as a two-dimensional array of coefficients representing different frequency components of the original video data. As is known, the DC component of DCT-transformed video data exhibits the highest level and the coefficients representing different frequency components that vary over a frequency band from lower to higher frequencies are of decreasing levels. Generally, the coefficients associated with the higher frequency components are of relatively low value. This is because of the inherent correlation exhibited by a video signal. Most of the information needed to reproduce a video picture of acceptable visual quality resides in the DC and lower frequency components of the DCT-transformed data.
To provide further data compression of the encoded video signal, the frequency-axis data, that is, the DCT coefficients, are encoded in a variable length code, such as the well-known Huffman code. Still further, to provide good noise immunity when magnetically recording the encoded video data, error correction encoding techniques are used, such as the Reed Solomon code. However, it has been found that, when data produced by DCT transformation or other orthogonal transformation is encoded in a variable-length code, the amount of data which represents the video information of one frame may differ substantially from the amount of data which represents the video information in another frame. That is, by using such variable-length coding, the data length of one frame may be much shorter than the data length of another. This presents a practical difficulty when attempting to edit a digital video tape that has been recorded with such variable-length encoded data.
One proposal for solving this problem proceeds by quantizing the DCT-transformed data by a particular quantizing step and then "rounding off" the quantized DCT coefficients so as to effectively eliminate those quantized coefficients of relatively low value. Quantization is achieved by mathematically dividing a DCT coefficient by a particular divisor. The greater the value of the divisor, the larger the quantizing step, resulting in coarse quantization. Moreover, this proposal for quantizing the DCT coefficients utilizes a non-uniform quantizing step, or divisor, within a two-dimensional DCT array. That is, divisors of a greater magnitude (or larger quantizing step) are used to quantize the coefficients for the higher frequency components and divisors of smaller magnitude are used to quantize the coefficients for the lower frequency components. Since the contribution to a video picture from the higher frequency components are not readily noticeable, coarse quantization of such higher frequency components generally does not result in a noticeable degradation of the picture. Accordingly, by using this variable quantization approach, DCT coefficients of higher frequency components are more highly compressed than the DCT coefficients of the lower frequency components. Stated otherwise, higher frequency data is more strongly compressed than lower frequency data.
This variable quantizing of the two-dimensional array of DCT coefficients can be thought of as being carried out by a quantizing unit having different divisors. If a two-dimensional array of DCT coefficients is thought of as being partitioned into 16 areas, these different areas may be depicted as areas 0, 1, . . . 15 of FIG. 1. The "horizontal" and "vertical" axes represent increasing frequencies in the horizontal and vertical directions, respectively. A single quantizing unit exhibits different quantizing steps, or divisors, for the respective areas into which the two-dimensional array is partitioned. Typically, 16 different quantizing units may be provided, with each such unit exhibiting a different quantizing step, or divisor, for each of the 16 areas. FIG. 2 is a schematic representation of quantizing units 0, 1, . . . 15 and further represents the quantizing step, or divisor value for each area shown in FIG. 1, depending upon which quantizing unit is selected. For example, if quantizing unit 2 is selected, the DCT coefficients in areas 0-3 are divided by the divisor 4, the DCT coefficients in areas 4 and 5 are divided by the divisor 6, the DCT coefficients in areas 6-10 are divided by the divisor 8, the DCT coefficient in area 11 is divided by the divisor 10, the DCT coefficients in areas 12, 13 and 14 are divided by the divisor 16 and the DCT coefficient in area 15 is divided by the divisor 32. FIG. 2 demonstrates that the higher frequency coefficients are divided by larger divisors, resulting in coarser quantization.
Quantization with the divisors shown in FIG. 2 have been implemented by multipliers. That is, rather than dividing a DCT coefficient by a divisor, the DCT coefficient is multiplied by a reciprocal of the divisor, that is, by a fraction. Although multipliers are simpler to construct than dividers, the use of a multiplying device generally adds to the complexity and size of the hardware and results in an increase in the cost of the encoding apparatus.
Relatively simple division of the value of a digital signal can be obtained by use of a shift circuit. It is known that the value of digital data can be divided by 2 simply by shifting that digital signal one place to the right. A division by 4 can be obtained by shifting digital data two places to the right, a division by 8 can be obtained by shifting the digital data three places to the right, and so on. However, although simple shift circuits thus can be used to achieve high speed, low cost division, such shift circuits generally are limited to performing division by the factor 2.sup.N. Consequently, quantization by dividing DCT coefficients by a number that is not a power of 2 cannot be performed by such simple shift circuits. That is, quantization by using the different divisors shown in FIG. 2 cannot be attained. Although quantization can be approximated by dividing a DCT coefficient by the value 2.sup.N, the accuracy obtained by such approximation is not satisfactory.